.\" $OpenBSD: arc4random.3,v 1.34 2014/07/19 16:11:16 naddy Exp $
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.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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.\" Manual page, using -mandoc macros
.\"
.Dd July 30, 2015
.Dt ARC4RANDOM 3
.Os
.Sh NAME
.Nm arc4random ,
.Nm arc4random_buf ,
.Nm arc4random_uniform
.Nd random number generator
.Sh SYNOPSIS
.In stdlib.h
.Ft uint32_t
.Fn arc4random "void"
.Ft void
.Fn arc4random_buf "void *buf" "size_t nbytes"
.Ft uint32_t
.Fn arc4random_uniform "uint32_t upper_bound"
.Sh DESCRIPTION
.Pp
These functions use a cryptographic pseudo-random number generator to generate
high quality random bytes very quickly.  One data pool is used for all
consumers in a process, so that consumption under program flow can act as
additional stirring.  The subsystem is re-seeded from the kernel random number
subsystem on a regular basis, and also upon 
.Xr fork 2 .
.Pp
This family of functions provides higher quality random data than those
described in
.Xr rand 3 ,
.Xr random 3 ,
and
.Xr rand48 3 .
They can be called in almost environments, including
.Xr chroot 2
and their use is encouraged over all other standard library functions for
random numbers.
.Pp
.Fn arc4random
returns a single 32-bit value.
.Pp
.Fn arc4random_buf
fills the region
.Fa buf
of length
.Fa nbytes
with random data.
.Pp
.Fn arc4random_uniform
will return a single 32-bit value, uniformly distributed but less than
.Fa upper_bound .
This is recommended over constructions like
.Dq Li arc4random() % upper_bound
as it avoids "modulo bias" when the upper bound is not a power of two.
In the worst case, this function may require multiple iterations
to ensure uniformity.
.Sh RETURN VALUES
These functions are always successful, and no return value is
reserved to indicate an error.
.Sh SEE ALSO
.Xr rand 3 ,
.Xr rand48 3 ,
.Xr random 3 ,
.Xr random 4
.Sh HISTORY
The original version of this random number generator used the RC4 (also known
as ARC4) algorithm.  In OS X 10.12 it was replaced with the NIST-approved AES
cipher, and it may be replaced again in the future as cryptographic techniques
advance.  A good mnemonic is
.Dq A Replacement Call for Random .
